The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
A down-hole hammer typically comprises a drill shank and a percussive drill head assembly at the working end of the drill shank. The percussive drill head assembly incorporates a drill bit. The drill bit typically comprises a drill bit head defining a cutting face and a shank extending from the drill bit head away from the cutting face. The drill bit is retained in a drill bit retaining means commonly referred to as a drive sub. The shank of the drill bit incorporates splines which cooperate with counterpart splines on the drive sub, the arrangement being that rotational torque applied to the drive sub is transmitted to the drill bit through the mating splines while allowing reciprocatory movement of the drill bit in response to percussive blows delivered by an air-operated percussive hammer. The splines are arranged to provide channels along which exhaust air from the percussive hammer can flow. The drill bit retaining means further comprises a retaining flange provided on the shank. The retaining flange is adapted to be engaged by a retaining ring for retaining the drill bit within the drive sub while permitting reciprocatory movement of the drill bit in response to percussive blows from the percussive hammer.
In operation of the down-hole hammer, the drill bit is subjected to high energy percussive impacts and rotational torque loads which are delivered through the shank. In particular, the high energy percussive impacts are delivered to the strike end of the shank and the high rotational torque loads are delivered through the splines provided on the shank.
The continual loadings on the drill bit through the drill shank can lead to material fatigue, a consequence of which is that the drill bit can fracture in the region adjacent the intersection between the shank and the drill bit head. This is most undesirable, as not only does the fractured drill bit need to be discarded and replaced, but also there is time involved in withdrawing the drill shank for the replacement of the fractured drill bit. Further, there may also be lost time if the drill bit head separates from the down-hole hammer, thereby requiring it to be retrieved separately from the bore hole.
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.